Plastic discs having a pit optical track structure are known as compact discs. Double sided discs in which information is recorded on two sides of such a structure formed by binding two compact discs and apparatus for playing such discs is also known, as in U.S. Pat. Nos. 4,903,140; 4,939,011; 5,097,465; 4,811,324. In forming such disks it is also taught in U.S. Pat. No. 5,284,538 and 5,378,517 that it is advantageous to bring the two disc portions together at an angle in order to facilitate bonding.
The term compact disc as used in this application includes any disc shaped recorded medium having pits and lands forming an optical track structure on a data information recording planar surface. Thus it includes such discs where the information stored as pits and lands is digital information, analog information, or information of a graphic or holographic nature. Examples of the different types of information storage are as follows: Digital information may be encoded into the pits and lands as pits of quantized lengths interpretable as a stream of digital data, wherein the length of each pit or land segment may be, for example, related to the number of 1's or 0's in a data stream. An example of such a system is the compact disc ("CD") audio or video disc. Analog information may be encoded into the pits and lands as pit lengths that may vary over a continuum of values, wherein the length of each pit or land segment may be, for example, related to a voltage level in a desired output. An example of such an analog system is the laser disc video recording system. Graphics may be placed on the surface of a disc by employing the light reflection properties of pitted surfaces that are visible to the unaided eye. Similarly holographic information may be stored in a pit/land format, particularly where it is possible to vary the pit depth to create reflection interference patterns that are visible as a holographic image viewable by the unaided eye.
In order to increase the density of information stored on compact discs it is known to manufacture a sandwich of two thin data bearing surfaces that are bonded together. It is necessary to use thin surfaces in order to minimize the coma associated with the passage of light through the plastic material to reach the data pits. It is also known to bond one thin data surface to a thin supporting surface. The two surfaces are bonded together, typically with the use of ultra-violet curable adhesives. The injection molding process and the ultra-violet bonding process tend to introduce distortions from planarity of the surface of the resulting compact disc, primarily because of the heat introduced by those processes. Thus when played the surfaces do not remain a fixed distance from the reading laser. Since the laser light is neither perfectly columnar nor uniform in intensity, the result is inconsistency in the reading of the data from the disc. This problem defeats the use of the conventional assembly line for the manufacture of compact discs. The conventional assembly lines did not face the aforesaid problems because the greater thickness of the plastic surface of the compact disc provided greater stability against deformation.
The use of two surfaces bonded by an adhesive enhances distortions in the surface. This is particularly true when one of the two thin discs has recently emerged at relatively elevated temperature from an injection molding machine while the second disc to be part of the "sandwich" is cooler. In the prior art practiced by such machines as those produced by Origin and Kitano, a uniform spread of the adhesive is obtained by laying down a circular bead of adhesive near the central hole of a first disc surface, bringing the second surface above the first until it contacts the bead of adhesive and then rapidly rotating the combination. Centrifugal force then spreads the adhesive uniformly between the two surfaces. This sandwich is then uniformly illuminated by ultra-violet radiation to cure the adhesive. The result is generally unsatisfactory as the surface is subject to deformation from planarity at each stage. It has been discovered by the present inventor that one cause of this deformity is that the rotation of the surfaces while they contact the adhesive tends to draw the outer circumference of the discs together. One aspect of the invention is designed to compensate for this defect in the prior art.